Children as young as seven are posting abusive comments and making threats against teachers on social media websites, it has been claimed.

A new survey suggests that pupils, and parents, are using these networks to insult school staff, make allegations of inappropriate behaviour and to comment on performance in the classroom.

In one case, a teacher reported that they had been racially abused, while another said that comments had been made about their sexuality.

But many teachers are afraid to report online abuse, often because they did not think anything could be done, or they did not think it would be taken seriously, the survey suggests.

The poll, conducted by the NASUWT teaching union, questioned more than 7,500 teachers about the use of technology.

It found that more than a fifth (21%) of those surveyed said that negative comments had been posted about them on social media sites.

Of these, almost two thirds (64%) said that the comments had been made by pupils, more than a quarter (27%) said that they had been posted by parents and the rest said that these had been made by both pupils and parents.

Just over three in five (61%) said that the pupils posting the comments had been aged between 14 and 16, with a third (35%) saying these youngsters were between 11 and 14.

Around a fifth (21%) said that the youngsters responsible were 16 to 19, while 3% said that they were between seven and 11.

Of those subjected to offensive posts made by pupils, the most common type of abuse was comments about their teaching - followed by insulting comments, or videos and photos taken without their consent.

Around 4% had faced allegations of inappropriate behaviour, and the same proportion had experienced threats.

The most common types of abuse by parents were also comments about their performance in the classroom and insulting remarks.

Asked to give examples, one teacher said a parent had told them they were "a nasty teacher, telling me to go 'home'".

Another said: "I had a video taken from the corridor while discussing a report card with another pupil. I was then accused of calling the pupil a bitch when I did not".

And a third told researchers that they had faced a false allegation that they had punched a pupil.

More than half (58%) of those surveyed did not report abuse from pupils, with 64% saying this was because they did not think anything could be done about it.

Of those that reported comments made by a pupil to a headteacher, around 40% said that no action was taken against the pupil.

Around 23% of those who reported pupils to the police saw action taken, along with 39% who reported pupils to social networking sites, the NASUWT claimed.

The union's general secretary, Chris Keates, said that technology had transformed the lives of teachers and pupils, but that steps need to be taken to protect teachers from abuse.

"Teachers are often devastated by the vile nature of the abuse they are suffering", she said.

Ms Keates added: "Schools need policies which prevent abuse and identify sanctions which will be taken against parents and pupils who abuse staff in this way.

"Schools should also be supporting staff in securing the removal of the offensive material from social media sites and encouraging the staff concerned to go to the police."

:: The NASUWT questioned around 7,500 teachers between February 21 and March 10.